1. Field of the Invention
The present invention relates to a microwave oscillation element (or microwave oscillator). Particularly, the present invention relates to a microwave oscillation element that is used for irradiating a microwave to a magnetic recording medium and writing data on an irradiated part, and relates to a thin film magnetic head using the same. Additionally, it is possible for the microwave oscillation element of the present invention to be applied to various types of high-frequency devices.
2. Description of Related Art
Regarding to high density of magnetic recording devices, and specifically hard disks incorporated therein as recording mediums, there is a demand for improvement in the performance of thin film magnetic heads.
In the thin film magnetic heads, a composite type thin film magnetic head is widely used. The composite type thin film magnetic head has a structure where a reproducing head having a magneto-resistive effect element (MR element) for reading and a recording head having an induction-type electromagnetic conversion element for writing are layered.
In magnetic recording, a recording medium (a hard disk) that is a recording object, is configured with discontinuous mediums of magnetic microparticles, and each magnetic microparticle has a single magnetic domain structure. In the magnetic recording, recording is executed with a plurality of the microparticles.
In order to increase recording density, asperities on boundaries in recording regions must be decreased. Therefore, the magnetic microparticles must be made small; however, this causes in a problem that heat stability deteriorates due to a decrease in the volume of the microparticle.
KuV/KBT gives an indication of the heat stability. Herein, Ku indicates anisotropic energy of the magnetic microparticle, V indicates a volume of one magnetic microparticle, KB indicates the Boltzmann constant, and T indicates an absolute temperature. Making the magnetic microparticle smaller means making V small, and this results in decreasing a value of KuV/KBT. Consequently, a measurement to be considered is to increase Ku. However, if Ku is increased, coercivity is increased. On the other hand, a writing strength of a magnetic head (the size of a magnetic field made by a magnetic head during recording) is determined by a saturation magnetic flux density of a soft magnetic material of a core. For that, when the coercivity of the recording medium increases over a tolerance value determined by a limitation of the writing magnetic field strength, it is impossible to write information.
As a first method to solve such a problem, use of a patterned medium is considered. In ordinary magnetic recording, plural magnetic microparticles N are used to record one bit. When one bit is recorded in one region having a volume of NV, heat stability is obtained by KuVN/KBT and is drastically improved.
As a second method to solve the problem of the heat stability, another method is proposed. Even though the method requires the use of a magnetic material having a large amount of Ku, in this method, recording is executed under the situation where the coercivity of the recording medium is decreased due to the application of heat just before the application of a writing magnetic field. This is referred as a heat assisted magnetic recording. This art is similar to light magnetic recording. However, the difference is that, while light has a space resolution in the light magnetic recording, a magnetic field has a space resolution in the heat assisted magnetic recording.
However, both of the first and second methods described above require significantly changes to a conventional magnetic head structure and a recording medium structure, resulting in large difficulties in technology and cost.
Under such a situation, Jimmy Zhu from Carnegie Mellon University has proposed a microwave assisted recording (document 1: IEEE TRANSACTIONS ON MAGNETICS Vol. 44, No. 1, January 2008, pp. 125-131) as a third method.
This method is realized when, in a conventional structure of a writer head, an MR element such as a TMR or CPP-GMR element is inserted between a main magnetic pole and a trailing shield thereof. It can be said that realizing this structure is much easier than manufacturing the patterned medium and the heat assisted recording head, and recently this has received much attention.
Also, in document 2: WO 2003/010758 (WO 2003/010758, JP National Publication No. 2005-525663) and document 3: JP Laid-Open Publication No. 2005-285242, similar microwave assisted magnetic recordings is disclosed.
In the microwave assisted magnetic recording proposed therein, a spin wave excitation element is used. The spin wave excitation element is configured with a magnetization free layer, a nonmagnetic layer, a magnetization pinned layer and a pair of electrodes. The magnetization free layer in which a magnetization direction changes according to an external magnetic field and the magnetization fluctuates, is formed in the vicinity of a recording magnetic pole. The nonmagnetic layer is layered on the magnetization free layer. The magnetization pinned layer is layered on the nonmagnetic layer, and the magnetization of the magnetization pinned layer is pinned. The pair of electrodes is formed at both edge parts in a lamination direction of a multilayer. When, in the spin wave excitation element, current is applied in a direction perpendicular to a surface of each layer of the multilayer, this applied current transfers an electron spin. Due to the transfer of the spin, a spin torque is generated, and due to the spin torque, a spin precession is excited on the magnetization free layer. In other words, due to spin polarized current injected from the magnetization pinned layer to the magnetization free layer by current flowing, the spin precession is excited on the magnetization free layer.
From the magnetization free layer in which the spin wave is excited, a high-frequency electromagnetic field in the microwave region leaks and the magnetization of the magnetic recording layer of the magnetic recording medium receiving the electromagnetic field fluctuates. As a result, magnetization inversion of the magnetic recording layer, which used to be impossible to achieve only with a writing magnetic field from the main magnetic pole, becomes possible.
In this case, a frequency of the high-frequency electromagnetic field, i.e., a frequency of the precession of magnetization of the magnetization free layer should be tuned to an inherent magnetic resonant frequency of the magnetic recording layer. In order to achieve this, it is required to adjust a thickness of the magnetization free layer, a magnetic field (a bias magnetic field) applied to the magnetization free layer, a spin polarized current amount (current amount for exciting the spin wave) injected to the magnetization free layer and the like.
In document 1 (IEEE TRANSACTIONS ON MAGNETICS Vol. 44, No. 1, January 2008, pp. 125-131), which is described above, art is disclosed. In the art, adjusting an amount of a perpendicular magnetic anisotropy of a magnetic layer, the magnetic layer named as “Layer with perpendicular anisotropy” and contacting the magnetization free layer, is relevant to effectively be given the bias magnetic field, and the frequency of the precession can be adjusted.
However, with only the contents of the art disclosed in documents 1 through 3 described above, it can be said that it is difficult to obtain stable oscillation even at high frequency. Moreover, it can be said that an improvement in an oscillation efficiency is not particularly expected.
On the other hand, in document 4: K. Yoshida, SRC (Head) digest (2008), it is reported that, when a CoIr alloy that is a material having negative Ku is used for a free layer as an oscillator, the stable oscillation is obtained even at high frequency. However, in document 4, a detailed description regarding a spacer layer and a material of an interface are not given, and it can be said that it is not clarified whether or not a spin injection having high efficiency is substantially executed. Additionally, it is known that a CoIr alloy is a material indicating negative AMR (document 4: T. R. McGuire et. al., IEEE TRANS. ON MAG., VOL. MAG-20, NO. 5, SEPTEMBER 1984). Therefore, it can be said that CoIr is a material indicating minority spin conduction (spin asymmetry coefficient β is negative).
Under the current situation where such a conventional art is disclosed, an inventor of the present application conducted an additional experiment of a combined element configuration, i.e., an experiment where a TMR/CPP-GMR element was used as the microwave oscillation element. In the TMR/CPP-GMR element, a CoIr layer that is a material where Ku is negative and the spin asymmetry coefficient β is negative, is applied to the magnetization free layer of the spin wave excitation element (microwave oscillation element) that is likely disclosed in the conventional art. As a result of this, contrary to the expectation of the inventor of the present application, the spin injection efficiency was low.
The present invention was invented in such a current situation. The object is to improve the efficiency of the spin injection and to provide a microwave oscillation element having extremely excellent oscillation efficiency.